This invention relates to methods of controlling corrosion in oil wells. In particular, it relates to such methods in which known anticorrosive agents are rendered more economically effective.
Crude oil, as it is produced from underground oil formations, is a mixture of various components including oil and from about 1 to about 95 percent water. The presence of water causes several problems for oil producers, one of the more significant being corrosion of metal in the oil production equipment.
When an oil well is drilled, steel pipe called "casing" is inserted into the hole and cemented in place to maintain the integrity of the hole. A second pipe, called the "production tubing" or simply "tubing", is then inserted into the well in the center of the casing, for withdrawal of the oil. At first, natural pressure in the formation may drive the oil through the tubing to the surface. However, once the natural pressure is dissipated, it becomes necessary to pump the oil to the surface. One typical means of pumping is to attach an electric pump to the bottom of the tubing. A more common means is to attach a mechanical pump to the bottom of the tubing with a rod (typically made of steel) called a "sucker rod" connecting the pump to a power source on the surface. The space between the casing and the tubing, called the "annulus", is used for introducing chemicals such as corrosion inhibitors to the well. The space inside the tubing contains the sucker rod (if one is used) and is the conduit for the oil being pumped to the surface. The bottom of the pump reaches into the collected borehole fluids, consisting of oil and usually also water at the bottom of the well and pumps the liquid to the surface, through the tubing.
Because failure of the casing can be difficult or impossible to repair (resulting in the need to drill a new well), and failure of the pump, tubing, or sucker rod can cost thousands of dollars in parts, labor, and lost production, control of corrosion is extremely important. In response to this need, numerous chemicals have been proposed for addition to the well fluids to prevent corrosion. In conventional practice, these chemicals are typically dissolved in a solvent, blended with a surfactant, and pumped into the annulus of the well. This may be followed with a "flush" of 1 to 000 liters of water. To the extent that an emulsion of the corrosion inhibitor in the flush water is formed, it will be an oil-in-water emulsion. Long term experience has taught that applications need to be frequent and if more than about one week apart (sometimes as short as 2-3 days apart) will lead to greatly increased corrosion. Even if the quantity of inhibitor is increased, the duration of the effective treatment is not increased. Further, these chemicals are expensive and excessive treatment can cause troublesome crude oil emulsions.
Another drawback of conventional treatments is that immediately after application the produced fluids have a very high (excessive) level of corrosion inhibitor. The level of corrosion inhibitor then rapidly falls so that by the next treatment the level is very low (insufficient). This irregular level of inhibitor leads to crude oil emulsions early in the cycle and corrosion in the later part of the cycle.
Therefore, it would be advantageous to provide a more efficient means of preventing corrosion in an oil well.
U.S. Pat. No. 3,732,166 (Lissant - Petrolite) discloses a method of cleaning wells by introducing a high-internal-phase-ratio emulsion (HIPRE) containing carbon disulfide (a paraffin solvent) to the well. This reference does not mention corrosion inhibiting compounds. U.S. Pat. No. 3,343,599 (Eddins - Petrolite) teaches reducing the porosity of oil and gas formations by applying a particular HIPRE to a well. This patent does not mention corrosion inhibiting compounds. U.S. Pat. No. 3,523,826 (Lissant - Petrolite) teaches a pipe cleaning "pig" made of a highly viscous HIPRE. At column 3, lines 43-45, it mentions that a corrosion inhibitor can be added. U.S. Pat. No. 3,617,095 discloses a system for transporting solids in a pipeline by using HIPREs. At column 3, lines 36-37, the possible inclusion of a corrosion inhibitor is mentioned. U.S. Pat. No. 3,490,237 discloses a thixotropic fuel made from a HIPRE. The patent discusses the inclusion of numerous specific corrosion inhibitors at column 3, line 66 to column 7, line 30. A brochure entitled "Questions and Answers--ENERSPERSE .TM." (bearing a 1989 publication date) vaguely describes an oil well corrosion treatment system in which one of the claimed benefits is the use of less surfactant to produce a dispersion of the corrosion inhibiting compound in water and the ability to tailor the treatment to the needs of each well.